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The invention relates to a process and to a device for eliminating the harmful volatile elements, in particular chlorides and/or sulfates, contained in a stream of particle-laden fumes, as well as to a use of said device in a plant for the production of cement clinker, in particular Portland cement clinker.
However, although more especially intended for such applications, the eliminating process and device according to the invention can also be used in all other fields in which it is wished to degas particle-laden fumes.
At present, in the field of cement production, a very widely used manufacturing process consists in performing the following steps:
finely grinding the mixture of raw materials required,
pre-heating the fines thus obtained in a cyclone exchanger to a temperature of between 800xc2x0 C. and 900xc2x0 C.,
firing the pre-heated material in a rotary tube furnace which heats said material to a temperature of over 1350xc2x0 C.
The material leaving the furnace is known as clinker. This is then cooled down and crushed to produce the cement.
It will be appreciated that the above-described process consumes considerable energy and that it may be advantageous to minimize the heat input by supplying the fumes given off in the furnace to the cyclone exchanger in order to provide the pre-heating.
This being said, it can happen that the raw materials contain mineral compounds that are volatile at high temperatures, in particular chlorides and/or sulfates. In apparatus in which the temperature is high, a cycle is then produced: said compounds are volatilized in the hottest area of the furnace, close to the clinker output, and condense for the most part at between 700xc2x0 C. and 950xc2x0 C., that is to say in the lower part of the cyclone exchanger.
As a result, said compounds cannot leave the furnace, either with the hot clinker, since they are in a volatile state, or with the partially cooled final fumes, since they are then condensed and accumulate, reaching very high concentrations in the material and in the gases that circulate within the firing apparatus.
At the points of condensation, they can thus lead to the adhering of the materials in powder form to the walls of said apparatus and cause obstructions prejudicial to the satisfactory production.
One known method of eliminating these harmful elements is to effect purging at the point at which they are in a gaseous form, that is to say in the lowest part of the exchanger, as soon the fumes of the furnace have left the latter. Such purging is generally referred to as xe2x80x98furnace fume by-passingxe2x80x99 The proportion of fumes that has to be purged depends on the quantity of volatile compounds; it can represent over 50% in certain cases.
For this purpose, a part of the fumes are sucked off and cool air is injected into them. The very rapid cooling of the gases caused by mixing with the cool air causes the volatile elements to condense, on one hand in the form of very fine grains and, on the other hand, in the form of deposits on the surface of the grains of suspended materials that have been sucked off with the fumes. There are thus obtained by-pass dusts constituted by a mixture of the raw materials undergoing transformation and of said condensed harmful elements. The dust-laden gases are then filtered to retain the dusts contained in said harmful elements.
The by-pass dusts represent, however, a production loss as they contain a part of the raw materials. By way of example, a by-pass rate of 10% of the fumes can lead to a production loss of 1 or more percent.
Furthermore, the by-pass dusts represent a cost to the environment as there is, most often, no suitable means of treatment and the dusts have to be disposed of. They thus represent additional heat consumption since they have received the energy necessary to decompose the carbonates contained in the raw materials. There is thus much at stake financially in a reduction of their quantity.
One solution to reduce the quantity of lost materials is to separate the dusts into two fractions, a coarse fraction poor in harmful elements, which can be reintroduced into the clinker manufacturing circuit, and another fine fraction rich in harmful elements which is eliminated, or which is upgraded separately.
Hitherto, this solution was used only with small fume extraction rates, of less than 5%.
However, as already stated, the fume extraction rate can sometimes be well above 5% and no solution for recycling has been proposed to date in this case.
The difficulties to be overcome are indeed numerous. The lower the flow rate of fumes extracted, the more intense the cooling, and the greater the proportion in the by-pass dusts of fine grains constituted by condensates. On the other hand, in the case of high extraction rates, the distribution of the harmful elements in the dusts is more random. Furthermore, in such a case, it would not be economical to use a solution that would cause the dusts to be collected together in a group of heterogeneous grain size distribution such as, for example, such as cyclone exchangers.
The object of the invention is to propose a process and a device for eliminating the harmful volatile elements, in particular chlorides and/or sulfates, contained in a stream of particle-laden fumes, as well as the use of said device in a plant for producing cement clinker, in particular Portland cement clinker, which remedy the aforementioned drawbacks and permit the recycling of at least a part of the particles poor in harmful elements extracted to eliminate the volatile elements, in the case of processes involving a high fume extraction rate.
Further objects and advantages of the invention will emerge in the course of the description that follows, and which is given only by way of illustration and is not intended to limit it.
The invention relates, first of all, to a process for eliminating the harmful volatile elements, in particular chlorides and/or sulfates, contained in a stream of particle-laden fumes, in which process there is extracted a given fraction of at least 5% of said fumes, referred to as xe2x80x98by-passed fumesxe2x80x99, the harmful elements of the by-passed fumes are caused to condense, at least with regard to their particles, according to a distribution that is a function of said fraction extracted, and said particles are filtered to isolate said harmful elements, characterized in that, at the time of filtering, said particles are separated into at least two fractions, one with a fine grain size and the other with a coarse grain size, using a separation selectivity adapted to said distribution of the harmful elements in order for said coarse grain size faction to be poor in harmful elements and for said fine grain size fraction to be rich in harmful elements.
The invention also relates to a device for eliminating the harmful volatile elements, in particular chlorides and/or sulfates, contained in a stream of particle-laden fumes, said device including means for extracting a given fraction of at least 5% of said fumes, referred to as by-passed fumes, means for condensing the harmful elements of the by-passed fumes at least with regard to their particles, according to a distribution that is a function of said fraction extracted, and means for filtering said particles to isolate said harmful elements, characterized by the fact that said filtering means include means for separating said particles into at least two fractions, one with a fine grain size and the other with a coarse grain size, said separating means having a selectivity adapted to said distribution of the harmful elements in order for said coarse grain size fraction to be poor in harmful elements and for said fine grain size fraction to be rich in harmful elements.
The invention further relates to the use of the device described above in a plant for producing cement clinker, in particular Portland cement clinker.